1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image of a photograph, a microfilm or the like on a light-receiving medium such as a photosensitive member.
2. Description of the Prior Art
Japanese Patent Application Publication No. 24187/1971 discloses an image forming apparatus for copying an original such as a photographic film, a microfilm or a drawing in an enlarged, one-to-one or reduced size. This apparatus includes a first projection optical system with a fixed lens for forming an aerial image of said original, and a second projection optical system for slit scanning said aerial image to re-focus said image on a photosensitive member. FIG. 1 shows said apparatus in a slit scanning sectional plane, namely in a plane perpendicular to the longitudinal direction of slit aperture, while FIG. 2 shows said apparatus in a section perpendicular to said slit scanning sectional plane. As shown in FIGS. 1 and 2, an image of a film 1 is formed by a projection lens 2 in the vicinity of two cylindrical Fresnel lenses 3, 4 which are superposed, as shown in FIG. 3, in such a manner that their generatrices mutually cross perpendicularly and their optical axes coincide with that of said projection lens 2. The cylindrical Fresnel lens 3 has a power in said slit scanning sectional plane while the cylindrical Fresnel lens 4 has a power in a plane perpendicular to the slit scanning sectional plane.
In the slit scanning sectional plane shown in FIG. 1, the cylindrical Fresnel lens 3 deflects principal rays R1 and R3, emerging from the projection lens 2, in the direction parallel to a principal ray R2, thus providing mutually parallel principal rays, after the Fresnel lenses, in said section as plane. Said principal ray R2 is parallel to and coincides with the optical axis of the projection lens 2. On the other hand, in the section perpendicular to the slit scanning sectional plane, shown in FIG. 2, principal rays R2, R4, R5 from said projection lens 2 are guided by the cylindrical Fresnel lens 4 toward the pupil of a copying lens 5. The aerial image of said film 1 is scanned and projected onto a photosensitive member 6 with displacement of said copying lens 5 in the direction of the arrow in the slit scanning sectional plane. The center a of said Fresnel lens 4, where the Fresnel angle is equal to zero, is positioned corresponding to the center b of an effective copying area.
On the other hand, in order to reduce the dimension of an image forming optical system for copying with slit scan, there is already known such an arrangement as shown in FIG. 4, in which an original and a photosensitive member are positioned optically conjugate, with the use of off-axis imaging of a copying lens. In said arrangement, the image of an original on a platen 7 is projected onto a photosensitive member 11 through two scanning mirrors 8, 9 performing slit scanning of said original by displacements to respective positions 8', 9' along said platen 7, and also through an in-mirror lens 10. In such arrangement, however, a principal ray R6 of the copying optical system is positioned oblique to the optical axis C of said in-mirror lens 10. Consequently, such slit scanning optical system utilizing off-axis imaging, if positioned to receive the light from the cylindrical Fresnel lenses 3, 4 shown in FIG. 1, is unable to guide the light to the photosensitive member 11.